Radio communication systems including a plurality of radio base stations forming their respective cells and radio terminal stations each belonging to any one of cells, in which each of radio terminal stations performs communication with a radio base station with its own cell, have been increasingly utilized.
In radio communication systems configured of cells, a plurality of cells using the same frequency band is adjoined to cause communication interference between cells.
As a method for solving this problem, it is effective to adopt a method for conducting communication using different frequency bands among radio base stations of adjacent cells causing interference therebetween (see Patent Document 1).
Using spatial multiplexing in communication between radio terminal stations in cells, IEEE802.11n (see Non-Patent Document 1) has been developing a wireless LAN system employing a MIMO (Multi Input Multi Output) technology which aims to provide high-speed radio communication in which each radio station (each radio base station or each radio terminal station) includes a plurality of antennas so as to receive/transmit data packets multiplexed using those antennas. Multiplexing transmission used in IEEE802.11n is performed via one-for-one correspondence between a radio base station (AP: Access Point) and a radio terminal station (STA: Station) such that transmitting data are distributed to a plurality of antennas and subjected to parallel transmission, i.e. spatial multiplexing transmission, thus improving a communication speed. To achieve this spatial multiplexing communication, radio base stations need to know the channel estimation result regarding channels between radio base stations and radio terminal stations in advance. Since the status of channels may fluctuate with respect to time, it is preferable to acknowledge the channel estimation result just before spatial multiplexing transmission.
IEEE802.11n has studied transmission beam forming as an option for improving a communication quality using the MIMO technology. FIG. 41 is a time chart illustrating a transmission operation of packet signals in transmission beam forming. The radio base station AP transmits a signal for requesting a channel estimation signal (or a sounding signal), so that the radio terminal station STA receiving it sends back the sounding signal. The radio base station AP receiving the sounding signal estimates a channel based on the sounding signal so as to perform transmission beam forming based on the estimation result, wherein data packets are transmitted using a plurality of antennas by way of a transmission beam which is directed in an appropriate direction.
Recently, an MU (Multi User)-MIMO technology, which can effectively utilize radio resources since a radio base station is allowed to perform one-for-multi communication with a plurality of radio terminal stations by spatially multiplexing the same radio channel in a radio system adopting the MIMO technology, has been developed (see Non-Patent Document 2). In the MU-MIMO technology, a radio base station performs communication by spatially multiplexing data packets destined to a plurality of radio terminal stations. FIG. 42 shows a time chart illustrating the operation of the MU-MIMO technology of Non-Patent Document 2 and a network configuration. Before transmitting packet signals, the radio base station AP carries out random-time carrier sense so as to transmit a call signal, including a destination address of packet signals, in an idle state preventing reception of radio signals. Radio terminal stations STA1, STA2, which are specified by the call signal, send back response signals. Upon receiving response signals, the radio base station AP estimates channels with the radio terminal stations STA1, STA2, so that the radio base station AP transmits packet signals to their destinations by use of a plurality of antennas installed therein while forming a plurality of beams in directions optimum for the radio terminal stations STA1, STA2 based on the estimation result.
In the illustration, the number of antennas installed in the radio base station AP is set to four while the number of antennas installed in the radio terminal station STA is set to two. Generally speaking, the maximum number of spatial multiplexes is equal to the number of antennas, so that the maximum number of spatial multiplexes of the radio base station AP is four while the maximum number of spatial multiplexes of the radio terminal station STA is two. The conventional MIMO technology carries out multiplexing transmission via the one-for-one correspondence between the radio base station AP and the radio terminal station STA, so that the maximum number of spatial multiplexes between the radio base station AP and the radio terminal station STA should be set to two due to a limitation imposed on the number of spatial multiplexes of STA; hence, the radio base station AP cannot demonstrate its full capability. In contrast, the MU-MIMO technology carries out multiplexing transmission via the one-for-multi correspondence between the radio base station AP and the radio terminal station STA, wherein the radio base station AP is able to carry out spatial transmission, using two spatial multiplexes, with each of two radio terminal stations STA so as to implement four spatial multiplexing transmissions in total, so that the radio base station AP is able to maximally demonstrate its capability.